In modern industrial production, valves are one of the indispensable key pieces of equipment. They act like "traffic police" in pipeline systems, precisely controlling the flow of fluids. Among the many types of valves, bellows sealed valves have become the first choice for demanding conditions such as flammable, explosive, and toxic environments due to their superior sealing performance and high safety. This article will delve into the working principles, structural composition, application fields, and the importance of bellows sealed valves in industrial safety, helping you gain a comprehensive understanding of this "safety guardian" in the industrial sector.
In traditional valve designs, packing seals are the most common sealing method. However, in some special conditions, such as those involving flammable, explosive, and toxic media, even the slightest leakage can trigger serious safety accidents. To overcome this challenge, bellows sealed valves were developed. They achieve zero leakage between the valve stem and the outside world through the dual sealing action of packing and bellows, significantly reducing safety risks and fortifying the safety line of defense in industrial production.
The core component of a bellows sealed valve is the bellows, a metallic tubular element with elastic deformation capabilities. When the valve is closed, the bellows is subjected to the pressure of the valve stem, undergoes elastic deformation, and tightly adheres to the valve seat, forming a reliable sealing barrier that effectively prevents media leakage. When the valve is opened, the valve stem lifts the bellows, allowing it to return to its original shape and enabling the medium to flow smoothly. This unique design ensures that bellows sealed valves maintain superior sealing performance in various harsh environments, including high pressure, high temperature, and corrosive media.
The structural design of bellows sealed valves is also very ingenious. It mainly consists of the valve body, upper valve stem, valve stem, bellows, lower valve stem, and pins. Both the upper and lower valve stems have a pair of helical track grooves, with two pins fixed on the valve stem and able to move freely within the grooves. When the lower valve stem is connected to the ball to control the opening and closing of the valve, the rotation of the upper valve stem does not cause the valve stem to rotate. Instead, through the action of the helical grooves, the valve stem only moves up and down. This design not only ensures the safe use of the bellows but also achieves precise control of the valve stem.
The main components of a bellows sealed valve include the valve body, bonnet, valve stem, and bellows. The valve body and bonnet are usually made of cast steel or stainless steel, materials known for their high strength and corrosion resistance, enabling them to withstand various harsh working conditions. The valve stem serves as the actuating mechanism for valve opening and closing, controlling the valve's on-off state through rotational or vertical movement.
The bellows is the key sealing element of the valve, made from a flexible metallic bellows material with good elasticity and corrosion resistance. Its lower end is securely welded to the valve stem, allowing it to stretch and contract stably with the valve stem's movement, thus maintaining the seal's integrity. Additionally, a V-shaped packing is installed at the upper part of the valve stem, providing a "double insurance" for the valve's sealing performance. Even if the bellows fails, the V-shaped packing can still function effectively to ensure the valve's sealing.
During the valve installation process, sealing gaskets are precisely installed on both sides of the fixed disc. With the tight cooperation of the valve cover and bolts, the gaskets are compressed to form a reliable sealing barrier. This multi-layered protection design ensures that bellows sealed valves perform more stably and reliably when facing complex and demanding working conditions.
Due to their superior sealing performance and high safety, bellows sealed valves are widely used in various industrial fields. In gate valves, globe valves, and safety valves, the primary role of the bellows is to enhance sealing performance and improve sealing capability, making them suitable for high-sealing-demand environments, such as those with toxic and polluting substances. In steam traps and pressure-reducing valves, the bellows serve as a sensitive element, increasing the valve's operational sensitivity and expanding its application range.
The most commonly used structural forms of bellows sealed valves are bellows globe valves and gate valves. Bellows globe valves are widely used due to their short opening and closing travel, which makes it easier to manufacture matching bellows. Bellows-sealed gate valves are suitable for small-diameter pipelines with Class 150 to 2500, operating temperatures ranging from -29 to 425°C (carbon steel) or -29 to 500°C (stainless steel). They are used to cut off or connect the medium in pipelines and feature low fluid resistance, a wide range of applicable pressures and temperatures, no restrictions on the direction of medium flow, and good sealing performance.
In various valve applications, bellows need to be selected based on their purpose and technical requirements. They can be categorized by layers into single-layer, double-layer, and multi-layer bellows; by manufacturing methods into integral and welded bellows; by shape into ring and helical bellows; and by assembly sequence into single and series-connected bellows. Each type of bellows has its own advantages and disadvantages and is suitable for different applications, requiring selection based on actual conditions.
In terms of material selection, bellows are commonly made from copper alloys, stainless steel, and plastics. The appropriate bellows material and connection method should be chosen based on different usage scenarios to ensure effective integration with the valve. For example, stainless steel bellows are the preferred choice for high-temperature, high-pressure conditions, while plastic bellows are more suitable for environments with high corrosion resistance requirements.
The safety and reliability of bellows sealed valves are their most prominent features. The bellows converts the dynamic seal of the valve stem at the packing gland into a static seal at the bonnet, making the valve's seal safer and more reliable. The lower end of the metallic bellows in a bellows valve is welded to the valve stem to prevent process fluids from corroding the valve stem, while the other end is placed between the valve body and bonnet to form a static seal. With suitable materials, bellows valves can be used in high-temperature (up to 500°C) and high-pressure (up to 2500Ib or 42MPa) conditions.
The upper flange of the bellows, together with the graphite gasket, bonnet, and bracket, forms a static seal that isolates the medium inside the valve body from the outside environment. In case of bellows rupture due to special circumstances, the packing gland at the upper part of the valve can still serve as an emergency backup. This multi-layered protection design ensures that bellows sealed valves perform more stably and reliably when facing complex and demanding working conditions.
The service life of bellows sealed valves is one of the important indicators of their performance. The Manufacturers Standardization Society of the Valve and Fittings Industry (MSS) SP-117 specifies the service life standards for bellows valves. The service life requirements for bellows valves vary under different working conditions and should meet the specific requirements of customers during design.
To ensure the long-term stable operation of bellows sealed valves, regular maintenance is essential. Before installation, valves with a working pressure greater than 1.0 MPa and those that serve as shut-off valves on main pipelines should undergo strength and tightness tests. Only those that pass the tests should be used. During the strength test, the test pressure is 1.5 times the nominal pressure, and the duration should be no less than 1 minute. The valve body and packing are considered qualified if there is no leakage. For the tightness test, the test pressure is 1.1 times the nominal pressure, and the duration should comply with the requirements of GB 50243 standard, with no leakage on the valve seat sealing surface being qualified.
Bellows sealed valves play a crucial role in demanding conditions such as flammable, explosive, and toxic environments due to their superior sealing performance and high safety. Through the dual sealing mechanism of packing and bellows, the valves achieve zero leakage between the valve stem and the outside world, significantly reducing safety risks. The core component, the bellows, is made from corrosion-resistant metallic materials, ensuring stable operation in high-pressure, high-temperature, and other harsh environments. In summary, bellows sealed valves, with their excellent sealing performance, high safety, and wide range of applications, have become an indispensable piece of equipment in modern industry. They not only ensure the safety of production activities but also effectively prevent environmental pollution, providing solid support for the sustainable development of industrial production.
